Gate controlled semiconductor components, such as normally off devices like MOSFETs (Metal-Oxide-Semiconductor Field-Effect Transistor) and IGBTs (Insulated-Gate Bipolar Transistor), or normally on devices like JFETs (Junction-gate Field-Effect Transistor), are commonly controlled by applying DC voltages to the gate. When a normally off device is used as a switch, turn-on of the component is accomplished by setting a gate electrode of the component to a positive voltage with respect to an emitter/source electrode. Similarly, when the component is turned off, a negative voltage with respect to the emitter/source is applied to the gate. For a normally on device like JFET, either a positive (P-type JFET) or a negative (n-type JFET) biased gate to source voltage is applied to turn off the device. Sometimes an opposite polarity gate voltage is used in on state to enhance the conductivity of the channel.
The above switching is carried out with a drive circuit. The drive circuit receives a switching command from a control circuitry or the like, which determines when the switch should be operated. The drive circuit further receives the above-mentioned positive auxiliary voltage Vcc and negative auxiliary voltage Vee. The zero voltage point between the auxiliary voltages is connected to the emitter/source of the controlled semiconductor component. The typical drive circuits thus drive the semiconductor component in response to the switching command by changing the potential of the gate with respect to the emitter/source of the component. When turn-on e.g. of an IGBT is commanded, the drive circuit applies positive auxiliary voltage Vcc to the gate, thereby making the gate-to-emitter potential to Vcc. Similarly, when the component is to be turned off, voltage Vee is switched to the gate and the gate-to-emitter voltage is made to be −Vee turning the component off. A series resistance, often called gate resistance, is connected between the voltage sources and the gate to limit the gate current to values safe for the drive circuit. A minimum value of the gate resistor is often given by the component manufacturer. This value reflects the fastest switching speed the component withstands without destruction and thus gives the lowest switching losses. In practice, it is common that a much higher value of gate resistor is needed to limit the RFI emissions caused by the high voltage and current change rates. This, is turn, creates high switching losses and lower efficiency of the equipment.
To achieve a better and more reliable control of the switching phenomena, the gate voltages are often made asymmetrical. Typical values in connection with IGBTs for positive and negative auxiliary voltages are +15 volts and −7 volts, respectively.